Nanotechnology Nanotechnology 29 (2018) 342001 (30pp) https://doi.org/10.1088/1361-6528/aac6ea Topical Review Heterogeneous photocatalysis and its potential applications in water and wastewater treatment: a review Syed Nabeel Ahmed1 and Waseem Haider1,2 1 School of Engineering & Technology, Central Michigan University, Mt. Pleasant, MI 48859, United States of America 2 Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI 48859, United States of America E-mail: [email protected] Received 7 March 2018, revised 3 May 2018 Accepted for publication 22 May 2018 Published 13 June 2018 Abstract There has been a considerable amount of research in the development of sustainable water treatment techniques capable of improving the quality of water. Unavailability of drinkable water is a crucial issue especially in regions where conventional drinking water treatment systems fail to eradicate aquatic pathogens, toxic metal ions and industrial waste. The research and development in this area have given rise to a new class of processes called advanced oxidation processes, particularly in the form of heterogeneous photocatalysis, which converts photon energy into chemical energy. Advances in nanotechnology have improved the ability to develop and specifically tailor the properties of photocatalytic materials used in this area. This paper discusses many of those photocatalytic nanomaterials, both metal-based and metal-free, which have been studied for water and waste water purification and treatment in recent years. It also discusses the design and performance of the recently studied photocatalytic reactors, along with the recent advancements in the visible-light photocatalysis. Additionally, the effects of the fundamental parameters such as temperature, pH, catalyst-loading and reaction time have also been reviewed. Moreover, different techniques that can increase the photocatalytic efficiency as well as recyclability have been systematically presented, followed by a discussion on the photocatalytic treatment of actual wastewater samples and the future challenges associated with it. Keywords: photocatalysis, heterojunction, nanomaterials, TiO2, water treatment (Some figures may appear in colour only in the online journal) Nomenclature Ebg Band gap AOPs Advanced oxidation processes g-C3N4 Graphitic carbon nitride BPA Bisphenol A GO Graphene oxide CB Conduction band IM Immobilized CDs Carbon dots LCA Life cycle assessment E. coli Escherichia coli MB Methylene blue 0957-4484/18/342001+30$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK Nanotechnology 29 (2018) 342001 Topical Review fi MC Methylthionine chloride below 20 nm, due to the nite size effect [13, 14]. This property, along with its photocatalytic ability, has been MF Microfiltration reported to be promising for water and wastewater treatment MNPs Magnetite nanoparticles technology by many researchers [15–18]. Similary, ZnO, a MO Methyl orange wide band-gap semiconductor, and g-C3N4, a metal-free NF Nanofiltration photocatalyst have recently emerged as promising materials for photocatalytic applications [19–22]. NPs Nanoparticles Another very important member of the photocatalytic PCOU P-coumaric acid family is graphene, which is formed by a two-dimensional PHA Phthalic anhydride array of hexagonal sp2 hybridized carbon atoms. Since its PMR Photocatalytic membrane reactors discovery in 2004 [23], graphene and it derivatives have gained significant attention in many applications, with pho- rGO Reduced graphene oxide tocatalytic nanocomposites being one of them. Graphene has S. aureus Staphylococcus aureus a unique two-dimensional structure, with high surface area, TEM Transmission electron microscopy high conductivity and electron mobility. Consequently, gra- phene can potentially promote electron–hole recombination TNH TiO2 nanoholes structure more effectively and thus improve the photocatalytic effi- VB Valence band ciency [24]. ZPC Zero point charge These semiconductors have the potential to degrade the contaminants and microorganisms from the water and waste water, through a process called heterogeneous photocatalysis. Heterogeneous photocatalysis utilizes photon energy and 1. Introduction converts it into chemical energy, and offers great potential for many applications specially water and waste water treatment. Clean and drinkable water, free of toxic materials, carcinogenic This process is really effective in degrading a wide range of substances and harmful bacteria, is necessary for human health. organic contaminants, eventually mineralizing them into According to the United Nations’ World Water Development carbon dioxide and water [25]. Therefore, unlike other water Report 2018, the demand for clean water is expected to increase treatment techniques such as coagulation and flocculation, by nearly one-third by 2050 [1].Cleanwaterisaprimary heterogeneous photocatalysis completely eliminates the con- requirement in a variety of crucial industries including electro- taminants, rather than transforming them from one phase to nics, food and pharmaceuticals. Therefore, in order to meet the another. water supply demands, more and more efforts are being made to develop new methods for the reclamation of wastewater, both from industries and households. Innovations in nanoengineering 2. Heterogeneous photocatalysis and nanotechnology promise great improvements in water quality with the help of nanosorbents, bioactive nanoparticles A fast-growing population and ever-improving industrializa- [2],nanocatalystsandnanoparticleenhancedfiltration [3].Atthe tion has rendered the problem of wastewater very vital in the nanoscale (1–100 nm),materialsdemonstratesignificantly dif- recent years, and consequently lead to researchers studying ferent physical and chemical properties from their bulk coun- the advanced oxidation processes (AOPs), and looking for terparts. Furthermore, a higher surface to volume ratio provides a solutions. Among the AOPs, semiconductor photocatalysis dramatic increase in the surface reactivity. As a result, nano- has emerged as a promising technique which has the potential technology-based products that decrease the amount of toxic for total mineralization of the organic pollutants [26–29] as substances to sub-ppb levels can help to achieve water quality well as toxic metal ions [30, 31]. Since the discovery of the standards and reduce the health concerns. photocatalytic properties of TiO in 1972 [32], there has been In the past decade, visible-light photocatalytic activities 2 lots of research devoted to the understanding of fundamental of many nanomaterials such as nanoparticles and quantum mechanisms and parameters in order to utilize this process for dots of CdS [4, 5], BiVO [6], nanoparticles and nanorods of 4 water and wastewater treatment applications. ZnO [7, 8], and nanocubes of AgCl [9, 10] have been The mechanism of heterogeneous photocatalysis is pri- reported. The most widely studied of these nanomaterials, marily described by the semiconductors’ capability to gen- TiO , has been around for more than three decades now and 2 erate charge carriers under light irradiation followed by the shows excellent photocatalytic activity, anti-reflection, as well generation of free radicals such as OH- which leads to further as self-cleaning ability [11]. In addition to this, TiO also 2 reactions, eventually forming CO and H O [33]. Therefore, possesses hydrophilicity [11], long term stability [11] and 2 2 the most attractive features of heterogeneous photocatalysis high photo-reactivity, along with lower toxicity and costs than include [34]: other semiconductors [11]. Furthermore, in order to provide magnetic separation to TiO2, embedding an Fe3O4 core has • The pollutants degrade into CO2 and other inorganic been proven very useful [12]. Fe3O4, being a photocatalyst substances completely. itself, possesses superparamagnetic behavior at particle sizes • The process takes place at ambient conditions. 2 Nanotechnology 29 (2018) 342001 Topical Review follows [42]: -+ Photoexcitation:TiO2 ++hehn ,() 1 -- Entrapmentoffreeelectrons:eeCB TR ,() 2 ++ Entrapment of holes:hhVB TR ,() 3 -+ - Chargecarrierrecombination:ehTR++ VB e CB heat, ()4 - - Photoexcitedelectronscavenging:() O2 ads +e O2 , ()5 -+ Oxidation of hydroxyls: OH+h OH ⋅ ,() 6 PhotodegradationbyOH- radicals:R - H +⋅¢⋅+OHR H2 O.() 7 The OH·radicals generated in equation (6) reduces organic impurities into intermediate compounds, which are further degraded by the same reaction until CO2 and water are released as by-products (equation (7)). The overall reaction can be summarized into the following steps [42]: Figure 1. Potential applications of heterogeneous photocatalysis. i. Mass transfer of the organic pollutants or bacteria from • The only requirement for the reaction to initiate is the the bulk liquid phase to the photocatalyst surface. presence of oxygen and ultra-bandgap energy, both of ii. Adsorption of the pollutant into the photon activated which can directly be obtained from the air and sun. photocatalyst surface. • It is possible to support the catalyst on various types of iii. The generation of OH·radicals and H2O2 followed by inert matrices which include glasses, polymers, carbon the chemical degradation of the contaminants. iv. Desorption of the intermediate or final products from nanotubes and graphene oxides [35–37]. the surface of the photocatalyst. • The catalyst is cheap, non-toxic and reusable. v. Mass transfer of the intermediate or final product into Figure 1 demonstrates the potential